In cryogenic air separation, unnecessary components such as moisture, carbon dioxide, a nitrogen oxide, or a hydrocarbon in feed air are previously removed, and then cryogenic air separation is performed. Recently, an adsorption method is used for this removal of unnecessary components.
This adsorption method is classified into a thermal swing adsorption method (TSA) and a pressure swing adsorption method (PSA).
In the thermal swing adsorption method, at least two adsorbers are provided and operated while switching between an adsorption step and a regeneration step. An adsorption step time and a regeneration step time correspond to each other. A regeneration step further includes a depressurizing step, a heating step, a cooling step, and a repressurizing step.
In an adsorber, a moisture adsorbent is packed at the air-inlet side, and a carbon dioxide adsorbent is packed at the latter part thereof. The aforementioned unnecessary components in air are continuously removed by alternately switching between a relatively low-temperature adsorption step and a relatively high-temperature regeneration step.
Moreover, as a moisture adsorbent, activated alumina, silica gel, K-A type zeolite, Na-A type zeolite, or Na—X type zeolite is used, and as a carbon dioxide adsorbent, Na—X type zeolite is used.
Meanwhile, when focusing on the adsorption of carbon dioxide in such a thermal swing adsorption method, since the cross-sectional area of an adsorption column is determined so that the air velocity at the inlet of an adsorbent layer is within a certain range, the determination of the amount of a carbon dioxide adsorbent to be packed in an adsorber, that is to say, the thickness of an adsorbent layer, has a practically important meaning.
Conventionally, the basic method for determining the thickness of an adsorbent layer has been as follows. When an air velocity and the cross-sectional area of an adsorption column are determined, by determining an adsorption step time, the amount x of impurities to be adsorbed in inflowing feed air is determined. The packed amount of an adsorbent is determined so that the total of the amount of impurities, which can be adsorbed in an adsorption equilibrium part, and the amount of impurities, which can be adsorbed in a mass transfer zone part, is equal to the amount x of impurities, or more.
That is, the sum of the length of an adsorption equilibrium part and the length of a mass transfer zone part in an adsorbent layer is considered to correspond to the thickness of the adsorbent layer (packed height). This design method is described in for example “Kaisetsu Kagaku Kougaku” page 190 to 195, written by Takeuchi et al., Baifukan, published Jan. 15, 1982 (see non-patent reference 1).
FIG. 2 illustrates the concept of such a conventional method, representing the change in concentration distribution of an adsorbed component proceeding in an adsorbent layer as a function of time. In this figure, the vertical axis represents a relative concentration of carbon dioxide in feed air. Also, the horizontal axis represents the packed height of an adsorbent layer which is normalized to be dimensionless.
The curves C respectively represent a mass transfer zone. C0 represents the mass transfer zone shortly after the beginning of adsorption, C1 represents the mass transfer zone at the point in time at which a certain time elapsed after the beginning of adsorption, and C2 represents the mass transfer zone after the elapse of more time.
Regarding the adsorption of carbon dioxide, it is known that the shape of the mass transfer zone C1 after a certain time elapsed is the same as the shape of the mass transfer zone C2 after the elapse of more time, and that the mass transfer zone proceeds while keeping a constant pattern.
FIG. 2 shows the mass transfer zone C2 when the front end thereof reaches the outlet end of an adsorbent layer. A time period from a point in time, at which feed air starts to be provided to an adsorbent layer, until a point in time, at which the front end of a mass transfer zone reaches the end of an adsorbent layer, is determined as an adsorption step time.
In FIG. 2, the region, in which an adsorbed component is saturated, (represented as the region M) is an adsorption equilibrium part. The total of the amount of an adsorbent in an adsorption equilibrium part and the amount of an adsorbent in the region, in which a mass transfer zone exists at this time (represented as the region N), is determined as the required amount of an adsorbent.
The reason why the amount of an adsorbent is determined in such a manner is that reducing the required amount of an adsorbent by increasing the utilization ratio η of an adsorbent is considered to be economically efficient. The relationship between an utilization ratio η and a mass transfer zone is represented by the following equation (2).η=1−fZa/H  (2)
Here, f represents a constant determined by the shape of a mass transfer zone, and is normally ½. H represents the packed height of an adsorbent, and Za represents the length of a mass transfer zone.
From equation (2), it is understood that the utilization efficiency is increased as the length Za of a mass transfer zone becomes shorter with respect to the packed height H of an adsorbent.
Meanwhile, in the removal of carbon dioxide in air, it is typical to select about 0.2 m/s as an air velocity. This is because the aforementioned utilization ratio of an adsorbent is increased by shortening the length of a mass transfer zone and increasing an equilibrium adsorption part due to the selection of a relatively slow air velocity.
However, in the case where scale-up is performed while keeping a fixed air velocity, from the relationship of “an air velocity=an amount of feed air/a cross-sectional area of an adsorption column”, the cross-sectional area of an adsorption column is increased in proportion with the amount of feed air. As a result, in a large-scale adsorber, the column diameter has to be larger than the packed height of an adsorbent.
In general, the distribution has to be considered so that feed air flows uniformly through the respective portions in an adsorbent layer. However, in the shape of an adsorber with a greater column diameter than the packed height of an adsorbent, it is difficult to uniformly flow feed air.
Also, the increase in the cross-sectional area of an adsorbent layer leads to the increase in the installation area of an adsorber. In order to decrease the installation area, for example, a radial flow adsorber and the like have been proposed.
In response to the demand to decrease the installation area of an adsorber, from the relationship of “an air velocity=an amount of feed air/a cross-sectional area of an adsorption column”, a solution of accelerating a feed air velocity can be considered. For example, when an air velocity of 0.1 to 0.2 m/s (under a pressurized air condition), which is conventionally considered as typical in the removal of carbon dioxide in air, is changed into 0.2 to 0.4 m/s, the cross-sectional area of an adsorption column is downsized to ½.
However, in a conventional adsorber, when an air velocity is simply accelerated in this manner, adsorbent particles are fluidized in an adsorbent layer, causing a big problem in that the adsorption operation becomes unfeasible.
The fluidization of adsorbent particles can be prevented by increasing the diameter of an adsorbent from a conventional value of about 1.5 to 1.6 mm to about 1.7 to 5 mm so as to increase the weight of one particle in the case of a spherical adsorbent called a bead. Also, in the case of a cylindrical adsorbent called a pellet, the corresponding diameter may be increased from a conventional value of about 1.5 to 1.6 mm to about 1.7 to 5 mm. Hereinafter, the diameter of a spherical adsorbent or the corresponding diameter of a cylindrical adsorbent is referred to as a particle diameter.
However, when the particle diameter of an adsorbent is increased, the adsorption rate is decreased and the length of a mass transfer zone is elongated. Therefore, the required packed height of an adsorbent layer is increased, and the amount of an adsorbent is also increased.
As a result, it is revealed that the downsizing of an adsorber is difficult in the method of accelerating a velocity of feed air flowing into an adsorbent layer on the basis of the conventional design method.
This type of problem is common among general gas adsorption such as the removal of volatile organic substances in air, as well as the removal of carbon dioxide in air. For example, this problem is recognized when carbon monoxide contained in high purity nitrogen is removed by using an inorganic porous substance in which metallic nickel is supported.
In addition, it is also recognized when impurities in a variety of gases are removed, such as when oxygen in an inert gas is removed by the oxidation reduction reaction of copper. Furthermore, it is also recognized when a trace amount of carbon monoxide in air is removed by a hopcalite catalyst or a catalyst in which a noble metal is supported by an inorganic porous substance.    [Patent Reference 1] Japanese Unexamined Patent Application, First Publication No. 2002-346329    [Non-Patent Reference 1] “Kaisetsu Kagaku Kougaku” page 190 to 195, written by Takeuchi et. al., Baifukan, published Jan. 15, 1982